As an alternative to Public Switched Telephone Network (PSTN) systems, cellular phone networks have proliferated over the last decades, where users with cellular phones have access to one or more networks at almost any location. Also a recent development is the wide spread use of Voice over IP (VOIP) telephony, which uses internet protocol (IP) over wired and wireless networks. With the availability of such diverse types of communication networks and devices capable of taking advantage of various features of these networks, enhanced communication systems bring different communication networks together providing until now unavailable functionality such as combining various modes of communication (e.g. instant messaging, voice calls, video communications, etc.). This technology is also referred to as unified communications (UC). A network of servers manages end devices capable of handling a wide range of functionality and communication while facilitating communications between the more modern unified communication network devices and other networks (e.g. PSTN, cellular, etc.).
Enhanced communication systems providing multi-modal communications operate in a similar fashion to (sometimes the same) data exchange networks where designated servers and their backups provide services (e.g. routing of calls). Session Initiation Protocol (SIP) is a commonly used communication protocol between components of such systems. Survivability is one of the critical requirements for enhanced communication systems. Communications may be interrupted if the data center goes out of service, one or more communication servers are taken out for planned or unplanned maintenance, or one or more inter-site Wide Area Network (WAN) links are down. Moreover, network failures including Domain Name System (DNS) failures, directory failures, authentication provider failures, and/or load balancing failures may cause interruptions, delays, and similar degradation of user experience.